Meanwhile, Russian President Vladimir Putin's decree recognising Crimea as a "sovereign and independent country" has triggered the toughest Western sanctions against Russia since the Cold War.

Washington and the European Union reacted to the referendum for Crimea to breakaway from Ukraine with asset freezes and travel bans. US President Barack Obama vowed to "increase the cost" if the Kremlin does not back down.

The US announced sanctions against seven Russian officials, including Rogozin, Mr Putin's close ally Valentina Matvienko who is speaker of the upper house of parliament and Vladislav Surkov, one of Mr Putin's top ideological aides. The Treasury Department also targeted Yanukovych, Crimean leader Sergei Aksyonov

In a show of solidarity with the White House, the European Unions's foreign ministers slapped travel bans and asset freezes against 21 officials from Russia and Ukraine following Crimea's referedum.

The 'Anschluss' of Crimea cannot rest without a response from the international community."

He was referring to Nazi Germany's forceful annexation of Austria.

Moscow, meanwhile, called on Ukraine to become a federal state as a way of resolving the polarisation between Ukraine's western regions - which favour closer ties with the 28-nation EU - and its eastern areas, which have long ties to Russia.

In a statement, Russia's Foreign Ministry urged Ukraine's parliament to call a constitutional assembly that could draft a new constitution to make the country federal, handing more power to its regions. It also said the country should adopt a "neutral political and military status," a demand reflecting Moscow's concern about the prospect of Ukraine joining NATO and possibly integrating closer politically and economically with the EU.

Russia is also pushing for Russian to become one of Ukraine's state languages alongside Ukrainian.

In Kiev, Ukraine's new government dismissed Russia's proposal as unacceptable, saying it "looks like an ultimatum".

Ukrainian Foreign Minister Andriy Deshchytsya visited NATO headquarters in Brussels to request technical equipment to deal with the secession of Crimea and the Russian incursion there.

Nato said in a statement that the alliance was determined to boost its cooperation with Ukraine, including "increased ties with Ukraine's political and military leadership".